Liquid fuel combustion with porous fiber burner

ABSTRACT

Improved combustion of liquid fuel is achieved with a porous fiber burner by forming a mixture of the vaporized fuel and all of the desired combustion air sufficiently heated to prevent condensation of the vaporized fuel, and introducing the heated mixture into the porous fiber burner to effect flameless combustion on the outer surface of the burner. The resulting surface combustion produces a high proportions of radiant heat and increased thermal efficiency while suppressing the formation of nitrogen oxides and other pollutants in the flue gas. Periodically, fuel gas may be supplied to the same porous fiber burner.

This is a continuation of application Ser. No. 378,109, filed May 14,1982, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to the combustion of liquid fuel with a porousfiber burner to achieve increased thermal efficiency and decreasedformation of atmospheric pollutants.

It is known that porous fiber burners for fuel gases yield high thermalefficiency while suppressing the content of nitrogen oxides (NO_(x)),carbon monoxide (CO) and unburned hydrocarbons in the combustionproducts or flue gas. With the growing need to limit the discharge ofpollutants into the atmosphere and with the pressure for greater thermalefficiency caused by high fuel costs, the use of porous fiber burnershas been given now impetus.

No simple, practical burner for liquid fuels has been available to givethe good results obtained by burning fuel gas with a porous fiberburner. Unfortunately, a porous fiber burner becomes inoperative when aliquid fuel is injected into it. Hence, there is great need to make thecombustion of liquid fuel as efficient and pollution-free as thatattainable with fuel gas.

Accordingly, a principal object of this invention is to improve thecombustion of liquid fuels to yield high thermal efficiency and lowemission of atmospheric pollutants.

Another important object is to carry out the combustion of liquid fuelsthrough the use of porous fiber burners.

Still another object is to provide simple and economic apparatus forconducting the combustion of liquid fuels with porous fiber burners.

A further object is to provide a combustion system with a porous fiberburner that can be readily switched from a supply of liquid fuel to asupply of fuel gas.

These and other objects and advantages of the invention will be evidentfrom the description which follows.

SUMMARY OF THE INVENTION

In accordance with this invention, the combustion of a liquid fuel isconducted by forming a mixture of the vaporized fuel and all of thedesired combustion air sufficiently preheated to prevent condensation ofthe vaporized fuel, and introducing the hot gaseous mixture into aporous fiber burner to effect flameless combustion on the outer surfaceof the burner. Besides producing considerable radiant heat, theflameless combustion of the originally liquid fuel yields a flue gaswith a low content of NO_(x), CO and unburned hydrocarbons. Generally,the NO_(x) content of the flue gas is below about 20 ppm (parts permillion).

While any vaporizable fuel may be used in accordance with thisinvention, for the usual residential or commercial heating plant theliquid fuel is a petroleum or other hydrocarbon fraction that can becompletely vaporized and has an end boiling point not exceeding about650° F. Preferably, the liquid fuel will have an end boiling point notexceeding about 500° F.

A feature of porous fiber burners is that all of the combustion air issupplied as primary air which rarely is more than 25% in excess of thestoichiometric requirement. Generally, not more than about 15% excessair is used in accordance with this invention.

The production of porous fiber burners is well known. Burners madeaccording to the teachings of U. S. Pat. No. 3,275,497 are preferred,particularly when such burners include aluminum powder in the porousfiber layer as disclosed in U. S. Pat. No. 3,383,159.

Vaporizing the liquid fuel can be carried out by passing it through theheated coil or tubes of a vaporizer or by spraying it into the hotcombustion air which initially must have a high enough temperature toensure complete vaporization of the liquid fuel. The liquid fuel canalso be preheated and then vaporized by injection into the hotcombustion air.

An important advantage of the liquid fuel combustion system of thisinvention is that the system can be simply and readily operated withfuel gas, such as natural gas, as efficiently and low in pollutantsemission as with liquid fuel. Simultaneous combustion of both a liquidfuel and a fuel gas is also possible without diminishing the thermalefficiency and suppression of pollutants formation characteristic offlameless surface combustion with a porous fiber burner.

Combustion air is preheated in any known manner, usually by indirectheat exchange with the hot flue gas from the combustion of the liquidfuel pursuant to this invention. When available, waste heat from anindependent operation may be used to preheat the combustion air or toaugment the amount of preheating. Temperature in degrees Fahrenheit(°F.) is a convenient measure of the amount of preheat in the combustionair and its admixture with vaporized liquid fuel. As previouslymentioned, the preheat temperature of the vaporized fuel-air mixturemust be high enough to prevent any condensation or misting of the fuelvapor. Even with a liquid fuel having a low boiling range, such asnaphtha, the preheat temperature of the vaporized fuel-air mixture willrarely be less than 150° F. and with a higher boiling liquid fuel, suchas kerosene, the preheat temperature of the vaporized fuel-air mixturewill generally be at least 400° F. A preheat temperature above 1000° F.is seldom justified economically and with some vaporized liquid fuelsmay cause preignition or flash-back before the mixture passes throughthe porous fiber layer of the burner.

BRIEF DESCRIPTION OF THE DRAWING

For further clarification of the invention, the ensuing description willrefer to the appended drawing which is a diagrammatic representation ofa combustion system operable in accordance with several preferredembodiments of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The drawing shows a combustion system in which furnace 10 is equippedwith porous fiber burner 11 in the bottom portion thereof. Combustionair supplied by blower 12 passes through pipe 13, heat exchanger 14,pipe 15, heater 16 and pipe 17 into pipe 18 which discharges into burner11.

Liquid fuel is fed by pump 19 through pipe 20 into heater 21 wherein theliquid fuel may be heated or even completely vaporized. The heated orvaporized fuel flows from heater 21 through pipe 22 into pipe 18 whereinit mixes with the preheated combustion air to form a hot, completelygaseous mixture which is supplied to porous fiber burner 11.

One way of starting the operation of this combustion system involvessupplying heat to the combustion air through heater 16 and to the liquidfuel through heater 21 so that the liquid fuel is completely vaporizedwhen it flows through pipe 18 in admixture with the preheated combustionair into burner 11. Any convenient source of heat, such as electricalresistance or combustion of a fuel, may be utilized in heater 16 andheater 21.

Upon igniting the preheated mixture of vaporized fuel and combustion airissuing at the exterior surface of porous fiber burner 11, flamelesssurface combustion takes place with the result that the entire exteriorsurface of burner 11 is substantially a uniform source of radiant heat.Water or other desired fluid flowing through coil 23 in furnace 10absorbs radiant heat from burner 11 as well as heat from the hotcombustion products or flue gas rising through furnace 10. The cooledbut still hot flue gas discharges from furnace 10 through pipe 24 intoheat exchanger 14 wherein it flows countercurrent to the combustion airsupplied by blower 12 and pipe 13 and thus transfers more of its heat tothe air stream. The further cooled flue gas leaves exchanger 14 throughpipe 25.

As the temperature in furnace 10 gradually rises from the time thatburner 11 is ignited, so will the temperature of the flue gas passingthrough exchanger 14 also rise until the combustion system reaches asteady state. During the period when temperatures in exchanger 14 areincreasing and the air stream is therefore receiving more and morepreheat, the amount of heat added to the combustion air by heater 16 maybe gradually diminished. Depending on the specific design of thecombustion system and its operation, when the steady state is reached,the flue gas passing through exchanger 14 may in some cases preheat thecombustion air sufficiently so that no further heat need be introducedinto the air stream during its passage through heater 16. In other caseswhere steady state operation does not furnish all of the desired preheatto the combustion air by indirect exchange with the flue gas leavingfurnace 10, the amount of preheat received by the air stream inexchanger 14 is supplemented by heat from heater 16.

Again, when steady state operation is attained, it is possible that thecombustion air preheated in exchanger 14 may be hot enough, with orwithout added heat from heater 16, to vaporize the liquid fuelcompletely when the fuel is diverted through pipe 26 and injected orsprayed into the hot air flowing through pipe 17. Further use of heater21 is then discontinued.

In the foregoing discussion of the combustion system of this invention,start-up of operation was achieved by introducing preheat intocombustion air through heater 16 and by heating the liquid fuel inheater 21. Alternatively, the start-up operation may be simply carriedout by supplying a fuel gas, such as natural gas or propane, throughpipe 27 to pipe 18 wherein it mixes with combustion air from pipe 17.The unheated mixture entering burner 11 is ignited on emanating from theouter surface of the porous fiber layer of burner 11. The resultingflameless surface combustion yields infra-red radiation and hot productsof combustion which gradually raise the temperatures of furnace 10, heatexchanger 14 and the combustion air flowing therethrough. As soon assteady state operation is attained, the preheated combustion air fromexchanger 14, with or without any additional preheat from heater 16 asmay be needed, is ready to be mixed with the liquid fuel instead of thefuel gas. At that time, pump 19 is put into operation to feed the liquidfuel to heater 21 or to pipe 26 for injection into pipe 17 if thecombustion air has sufficient preheat to vaporize the fuel. In eithercase, simultaneously, the flow of fuel gas from pipe 27 is stopped andthe hot mixture of combustion air and vaporized fuel flows through pipe18 into burner 11 to continue flameless surface combustion on its outersurface.

In describing the start-up operation of the combustion system by using afuel gas, it is obvious that the invention provides a combustion systemthat functions with liquid fuel or fuel gas or even a mixture of bothand delivers high thermal efficiency and low emission of pollutants inall cases. This is an outstanding advantage of the invention inasmuch asprior efficient combustion systems are usually designed to give optimumresults when burning only liquid fuel or fuel gas and hence the fuelsupply to such prior systems cannot be switched from liquid to gas orvice versa. When a fuel gas is available to start up the operation ofthe combustion system illustrated in the drawing, and the system isdesigned to preheat the combustion air through heat exchanger 14 to atemperature high enough that the liquid fuel is completely vaporizedjust by injection into the hot air stream, the system can be furthersimplified by the elimination of heater 16 and heater 21.

As an example of the invention, light naphtha with an initial boilingpoint of 110° F. and an end boiling point of 315° F. is selected as theliquid fuel. Porous fiber burner 11 in furnace 10 is designed for anominal firing rate of 80,000 BTU/h (British Thermal Units per hour).Naphtha is fed by pump 19 at the rate of 4 lb/h (pounds per hour) andcombustion air is supplied by blower 12 at the rate of 63 lb/h which isabout 5% excess air. Inasmuch as the combustion air leaves heatexchanger 14 at a temperature of 215° F., the naphtha at ambienttemperature of 70° F. is completely vaporized by spraying it into thepreheated air. The resulting mixture of vaporized naphtha and combustionair has a temperature of about 150° F. whereas the dew point of themixture is below 100° F.

The flameless combustion of the mixture on the outer surface of porousfiber burner 11 makes that surface a uniform emitter of infra-red energyand yields a flue gas with an NO_(x) content of 15 ppm. The content ofCO and unburned hydrocarbons is also very small.

In another example of the invention, kerosene with an initial boilingpoint of 305° F. and an end boiling point of 540° F. is heated to atemperature of 280° F. by passage through heater 21. Combustion airpreheated to a temperature of 350° F. is mixed with the preheatedkerosene in pipe 18 to provide 10% excess air and to effect completevaporization of the kerosene. The resulting mixture has a temperature ofabout 340° F. and a dew point of about 275° F.

The kerosene has a net heating value of 18,340 BTU/lb and is fed tofurnace 10 with a nominal heating capacity of 150,000 BTU/h at the rateof 9 lb/h which is a firing rate equal to 110% of the design capacity offurnace 10. Even with this firing overload, porous fiber burner 11 isuniformly radiant and yields a flue gas with an NO_(x) content of 17 ppmand very little CO and unburned hydrocarbons.

In still another example of the invention, diesel oil with an initialboiling point of 345° F. and an end boiling point of 660° F. ispreheated to a temperature of 365° F. in heater 21. Combustion air ispreheated to a temperature of about 460° F. and is mixed with thepreheated diesel oil to provide 15% excess combustion air. As a result,the mixture has a temperature of 445° F. and all of the diesel oil is invapor form. The dew point of the gaseous mixture is not more than 360°F.

Furnace 10 with porous fiber burner 11 having a rated heating capacityof 300,000 BTU/h is fed the vaporized diesel oil at the rate of 16.5lb/h, the oil having a net heating value of 18,240 BTU/lb.

Flameless combustion of the hot mixture on the outer surface of burner11 produces a high proportion of radiant heat and a flue gas with anNO_(x) content of 19 ppm. The flue gas also contains negligible amountsof CO and unburned hydrocarbons.

To summarize, burning a liquid fuel through the use of a porous fiberburner achieves several noteworthy advantages. The formation of NO_(x),CO and unburned hydrocarbons in the flue gas is suppressed. High thermalefficiency is attained not only because considerable radiant heat isgenerated but also because large amounts of excess combustion air areavoided. The combustion system of the invention provides greatoperational flexibility including variation of the liquid fuel and evenits replacement by fuel gas.

While the examples involve petroleum fractions as the liquid fuel fed tothe porous fiber burner in vapor form, similar hydrocarbon fractions canbe obtained from tar sands, shale oil, coal liquefaction and syntheticfuels. In the future, such other liquid fuels may be economicallyattractive. The same may also be possible with liquid fuels such asalcohols derived from agricultural products. Hence, the liquid fuelselected for combustion in accordance with this invention may varygreatly so long as it is completely vaporizable. Obviously, the liquidfuel must be free of any ash residue. The selected liquid fuel shouldalso have a low sulfur content to comply with the sulfur emissionlimitations of applicable ordinances.

It should be noted that heater 21 is preferably used in most cases tomerely preheat the liquid fuel rather than to completely vaporize it.There are too good reasons for this preference. The high temperaturerequired in heater 21 to completely vaporize the liquid fuel may causecracking and the formation of unvaporizable residues particularly withliquid fuels having an end boiling point exceeding about 500° F.Moreover, the large volume of combustion air which is mixed with theliquid fuel depresses the dew point substantially below the end boilingpoint of the liquid fuel. Thus, in the example using kerosene with anend boiling point of 540° F, the liquid fuel is preheated to atemperature of only 280° F. and yet is completely vaporized when mixedwith 10% excess air preheated to a temperature of only 350° F. becausethe resulting hot mixture has a dew point of about 275° F.

Many variations and modifications of the invention will be apparent tothose skilled in the art without departing from the spirit or scope ofthe invention. For example, if waste heat from an independent operationis available, it may be used to preheat the combustion air and heatexchanger 14 may be eliminated. When heater 16 is installed tofacilitate start-up of the combustion system and heat exchanger 14supplies all of the desired preheat to the combustion air when steadystate operation is attained, the flow of hot combustion air may bediverted from pipe 15 through a by-pass pipe directly into pipe 18.Also, part of the combustion air flowing in pipe 13 may be diverted to aby-pass pipe discharging directly into pipe 15 so that the temperatureof all the air in pipe 15 is adjusted to a desired temperature.Accordingly, only such limitations should be imposed on the invention asare set forth in the appended claims.

What is claimed is:
 1. The improved combustion of liquid fuel having anend boiling point not more than about 650° F. to achieve high thermalefficiency and low NO_(x) emission which comprises preheating thecombustion air in an amount not more than 15% in excess of thestoichiometric requirement, forming prior to combustion a hot,completely gaseous mixture of said liquid fuel and said amount ofpreheated combustion air, introducing said hot mixture into a porousfiber burner containing a uniformly distributed, small amount of finealuminum powder, effecting flameless combustion of said hot mixture onthe outer surface of said burner thereby yielding hot flue gas of lowNO_(x) content below about 20 ppm, and passing said hot flue gas incountercurrent heat exchange with said combustion air for the aforesaidpreheating thereof.
 2. The improved combustion of liquid fuel accordingto claim 1, wherein said liquid fuel is heated with applied heat to atemperature below its end boiling point prior to being mixed with thepreheated combustion air.
 3. The improved combustion of liquid fuelaccording to claim 2, wherein the combustion air is preheated bycountercurrent heat exchange with the hot flue gas to a temperaturebelow the end boiling point of said liquid fuel.
 4. The improvedcombustion of liquid fuel according to claim 1, wherein said liquid fuelis kerosene which is heated with applied heat to a temperature below itsinitial boiling point prior to being mixed with the combustion airpreheated by countercurrent heat exchange with the hot flue gas to atemperature below the end boiling point of said kerosene.
 5. Theimproved combustion of kerosene according to claim 4, wherein the amountof preheated combustion air is not more than 10% in excess of thestoichiometric requirement.
 6. The process for the improved combustionof liquid fuel having an end boiling point not exceeding about 650° F.by flameless combustion on the outer surface of a porous fiber burner,which comprises initially supplying a mixture of fuel gas and combustionair to said porous fiber burner into which a small amount of finealuminum powder has been uniformly distributed, effecting flamelesscombustion of said mixture on the outer surface of said burnercontaining said aluminum powder, passing the resulting hot flue gas incountercurrent heat exchange with said combustion air to preheat saidcombustion air to a predetermined high temperature, thereafter replacingsaid fuel gas by injecting said liquid fuel into said combustion air atsaid predetermined high temperature to form prior to combustion acompletely gaseous mixture in which the amount of said combustion air isnot more than 15% in excess of the stoichiometric requirement, supplyingsaid completely gaseous mixture to said burner containing said aluminumpowder, effecting flameless combustion of said completely gaseousmixture on the outer surface of said burner containing said aluminumpowder thereby yielding hot flue gas of low NO_(x) content below about20 ppm, and passing said hot flue gas of low NO_(x) content incountercurrent heat exchange with said combustion air in said amount ofnot more than 15% in excess of the stoichiometric requirement.
 7. Theprocess of claim 6 wherein the liquid fuel is heated with applied heatto a temperature below its end boiling point prior to injection into thecombustion air at the predetermined high temperature.
 8. The process ofclaim 7 wherein the predetermined high temperature of the combustion airis below the end boiling point of the liquid fuel.
 9. The process ofclaim 6 wherein the liquid fuel is kerosene and said kerosene is heatedwith applied heat to a temperature below its initial boiling point priorto injection into the combustion air at a predetermined high temperaturebelow the end boiling point of said kerosene.
 10. The process of claim 9wherein the amount of combustion air is not more than 10% in excess ofthe stoichiometric requirement.